Catheters have long been used in the medical field to invasively obtain patient information and administer treatment. A conventional catheter is an elongate tube having a distal end and a proximal end. The distal end is designed for insertion into a fluid-filled passageway or cavity in the patient, such as one of the various intravascular conduits. The proximal end of the catheter remains outside of the patient and is provided with a termination assembly accessible to the health care provider. In this manner, the catheter provides a communication link between the patient's fluid-filled passageway or cavity and the health care provider for diagnosis and treatment.
Typically, the catheter includes one or more axial conduits known as lumens extending between the distal and proximal ends of the catheter. These lumens may contain, for example, electrical wires or optical fibers that transmit information between sensors located at the distal end of the catheter and bedside instruments at the proximal end of the catheter. The operation of the sensors is controlled and their outputs interpreted by the bedside instruments, allowing the sensor/cathete/instrument system to be used for monitoring and diagnosis.
Other lumens may extend between the termination assembly and ports provided at various points along the catheter, placing the termination assembly of the catheter in fluid communication with those ports. As a result, characteristics of fluids in the patient passageway can be monitored and additional therapeutic fluids can be introduced by the health care provider.
One application in which catheters have been extensively used is the determination of volumetric flow rate in an intravascular conduit. In that regard, several catheter-based techniques have been developed to determine a patient's cardiac output, i.e., the volumetric flow rate of blood in the patient's pulmonary artery.
The first of these approaches is conventionally termed the "thermal dilution" method. Under this approach, a bolus of cold solution is introduced into one of several lumens in a multiple-lumen catheter, via the termination assembly. The cold solution then enters the intravascular conduit through a port at the end of the lumen and on the exterior of the catheter. A thermistor located on the distal, downstream end of the catheter is coupled to the termination assembly by wires positioned in another lumen. The "dilution" of, or change in, blood temperature at the thermistor with time is then measured by an instrument coupled to the termination assembly. The resultant thermal change is electronically interpreted and cardiac output computed therefrom.
Thermal dilution techniques also typically employ an inflatable segment or balloon at the distal end of the catheter. This balloon is coupled to the termination assembly by yet another lumen in the catheter. The balloon can, thus, be controllably inflated by the health care provider and used as a flotation device to facilitate positioning of the catheter in the pulmonary artery.
The thermal dilution method does, however, have certain disadvantages. For example, this technique has proved to be of limited accuracy. In addition, cumbersome apparatus are required to provide the bolus and only intermittent information can be obtained.
Another approach to the measurement of cardiac output involves the use of ultrasonic energy. Unlike the thermal dilution method discussed above, ultrasonic techniques can provide cardiac output measurements continuously. This is of considerable value in the treatment of critically ill patients whose cardiac functions may change abruptly.
Ultrasonic techniques involve the use of a transducer positioned close to the distal end of the catheter. This transducer is connected to the termination assembly at the proximal end of the catheter by electrical wires threaded through one of the catheter lumens. A bedside monitor attached to the termination assembly applies a high-frequency electrical signal (typically in the megaHertz range) to the transducer, causing it to emit ultrasonic energy. Some of the emitted ultrasonic energy is then reflected by the blood cells flowing past the catheter and returned to the transducer. This reflected and returned energy is shifted in frequency in accordance with the Doppler phenomenon.
The transducer converts the Doppler-shifted, returned ultrasonic energy to an output electrical signal. This output electrical signal is then received by the bedside monitor via the lumen wiring and is used to quantitatively detect the amplitude and frequency-shifted Doppler signal associated with the ultrasonic energy reflected from the moving blood cells.
Existing ultrasonic measurment systems process the amplitude and frequency shift information electronically to estimate the average velocity of the blood flowing through the conduit in which the transducer-carrying catheter is inserted. Such systems also require that an independent estimation of the crosssectional area of the conduit be made using one of a variety of techniques taught in the literature, including, for example, the approach disclosed in U.S. Pat. No. 4,802,490. Cardiac output is then computed by multiplying the average velocity and cross-sectional area estimates.
One particular method of generating and processing ultrasonic signals for use in cardiac output determination employs a cylindrical transducer. The transducer is mounted coaxially on a catheter suitable for insertion into the pulmonary artery. As will be appreciated, there is, thus, a need for a catheter capable of carrying a cylindrical ultrasound transducer. Furthermore, in order to enhance the clinical utility of such a catheter, the catheter should retain some or all of the clinical functions already available through nonultrasonic catheters. Such a multifunction catheter would, however, be subject to a variety of design constraints.
Specifically, the catheter must have a small diameter for insertion into the particular conduit of interest and to minimize trauma to the patient at both the point of entry and along the inside region of the conduit in which it is inserted. Furthermore, the catheter should be designed so that the transducer does not significantly alter the catheter's diameter, thereby minimizing flow occlusion, thrombus formation and other mechanical traumas.
The catheter should also have a high lumen count, in other words, a relatively large number of independent lumens, so that several types of information can be collected and a variety of treatments performed. In addition, the lumens should have relatively large cross sections, especially when they are employed to measure pressures via a fluid connection between the distal and proximal ends of the catheter. The construction of a catheter having a small diameter, however, is in direct conflict with the provision of a high lumen count and large lumen cross section.
The flexibility of the catheter should also be designed to provide an optimal balance between the catheter's maneuverability through tortuous passageways and its tendency to kink and fold. Furthermore, it is essential that the flexibility be uniform over the length of the catheter to further reduce the probability of kinking due to the forces applied on flexible sections of the catheter by other, relatively inflexible, sections during insertion.
Finally, because catheters are used invasively, they are conventionally disposed of after a single use. Thus, it is desirable to keep the catheter's unit cost as low as possible. For that reason, the construction of the catheter should be simple and involve a minimum expense. Further, the electrical and mechanical coupling of a transducer to the catheter should be straightforward.
In view of the preceding remarks, it would be desirable to provide a small-diameter multifunction catheter that is capable of carrying a coaxially mounted ultrasound transducer that has a high lumen count and large lumen cross-sectional area, and that is uniformly flexible and easy to construct.